The present invention relates to a heat insulating structure for low-temperature or cryogenic pipings and, more particularly, to such a heat insulating structure which is advantageously suited to heat insulation of pipelines transferring LNG or the like cryogenic fluids. An object of the present invention is to provide an improved heat insulating structure having an excellent heat insulating properties which is substantially free from cracking in heat insulating barriers thereof and which can be easily applied and affixed to pipes or other objects having curved surfaces.
Heretofore, many cryogenic pipings such as LNG pipelines have employed rigid polyurethane foams or the like rigid synthetic resin foams as their heat insulating materials. Such rigid synthetic resin foams have low thermal conductivity and are excellent in many other physical properties. However, such conventional rigid foams undergo large thermal expansion and shrinkage. For example, conventional rigid foams having a density of about 30 to 35 kg/m.sup.3 undergo dimensional shrinkage of about 2.5 to 3 percent when the temperature changes about 180.degree. C. (e.g., from room temperatures down to -160.degree. C.). Since such foams do not have flexibility or elongatability enough to accommodate to such a dimensional change without reluctance, cracks occur in heat-insulating barriers formed of such conventional rigid foams, resulting in reduced heat insulating properties of the barriers. One known solution to this drawback is to restrict the dimensional change of foams by embedding in the inner or outer surface portion or intermediate portion thereof glass meshes which undergo relatively small shrinkage at low or cryogenic temperatures. However, this solution also entails complicated working processes and economical demerits.
In the meantime, bending a board of conventional ridig foam to form the same into a shape fitting to the curvature of the surface of a pipe or the like object requiring heat insulation often causes cracks in the bent sections of the board of leaves there noticeable recesses or thickness loss due to a large compression strain. For solving this problem, such formed boards have had to be cut out from massive foams or they are prepared by expansion molding using molds appropriate to the sizes and shapes of the specific target boards. Thus, this method requires increased cost because of increased material quantity, equipment and manhour.